1. Field of the Invention
This invention relates to mine roof bolts and, more particularly, to a mine roof bolt which is positioned in a bore hole drilled in a rock formation in a mine roof.
2. Description of the Prior Art
It is a well established practice in underground mining work, such as coal mining, tunnel excavation, or the like, to reinforce or support the mine roof 1 of the mine to prevent rock-falls or cave-ins. The most common means presently used to support a mine roof is an elongated bolt 2 or bar that is inserted into the rock formation above the mine roof in a bore hole 3 and securely fixed within the bore hole by an anchoring device 4, such as a mechanical anchor, a quick-setting resin which surrounds the end of the bolt within the hole, or both. The roof bolt, is then placed under tension and is used to hold a metal support plate or bearing plate 5 in close engagement with the roof. See for example, U.S. Pat. No. 4,655,645.
The bore holes should be drilled perpendicular to the mine roof surface. See FIG. 1a. This minimizes bending of the bolt during installation, as well as minimizing the bending stress on the bolt. However, in practice, especially in inclined or uneven mine roof surfaces, the bore holes can be skewed with respect to the mine roof surface or the mine roof surface simply is not planar in the area of the bore hole. See FIG. 1b. This can cause bending of the bolt or abrasion of the bolt as it rubs against the perimeter of the bearing plate hole during installation, which results in high bending stresses or other forms of weakening on the bolt near the bolt head 6. In addition, excessive boom pressures from the hydraulic roof bolter equipment can place undue forces on the bolt head, particularly with an uneven roof. Typically, the boom exerts forces of between 3,000-6,000 pounds on the bolt head. In some cases, the bolt head snaps off the bolt shaft. Further, this skewed arrangement causes large amounts of friction between the rotating bolt head and the bearing plate resulting in an inefficient system with respect to the torque/tension relationship of the bolt. This is true even with a friction reducing washer 7.
It is an object of my invention to provide a mine roof bolt assembly that can compensate for a nonplanar or uneven mine roof or a bore hole not being perpendicular with the mine roof surface to provide adequate tensioning of the bolt system. It is further an object to provide an assembly which includes a member which functions as a low friction washer.